An article will give you a detailed understanding of class inheritance and polymorphism in Java

Inheritance of classes

##Subclasses and their definitions

The definition of a subclass uses the keyword extends

Format:

class SubClass extends SuperClass{
........
}

Subclasses can inherit the properties and methods of the parent class; subclasses cannot inherit properties with private modifiers , method; subclasses cannot inherit construction methods

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Single inheritance:

Java only supports single inheritance, that is, it can only inherit from one class, and there can only be one class name after the extends keyword.

Advantages: Possible conflicts between multiple parent classes can be avoided.

Interface interface mechanism allows one class to implement multiple interfaces

super keyword

The super keyword points to this key The parent class of the class where the word is located, the parent class reference variable can point to the subclass object

Format:

super.someNethod([paramlist])//调用父类中的someMethod（）方法

Creation of subclass object

Steps:

Allocate all memory space required by the object and initialize it to 0 value

According to inheritance relationship, top-down explicit initialization

According to inheritance relationship, top-to-bottom initialization Call the constructor method

Another expression of subclass object initialization:

Basic initialization, execute the constructor method, first execute the parent class constructor method, execute the parent class constructor Before the method, an explicit initialization statement of the parent class must be executed.

Overriding of methods

(1) The return value type of the overridden method in the subclass must be the same as the return value type of the overridden method in the parent class

(2) The access permissions of overridden methods in subclasses cannot be reduced

(3) Subclass overrides cannot throw new exceptions: method overriding is to implement object runtime polymorphism The basis of

Polymorphism: compile-time polymorphism and run-time polymorphism

Compile-time polymorphism: such as overloading

Run-time polymorphism : For example, rewrite

Upcasting

Convert a reference to an object of one type into a reference to an object of another type

Downward Casting (casting)

instanceofOperator

aOblectVariable instanceof SomeClass

##aOblectVariable # When ## is of type

SomeClass, the value of the expression is true, otherwise it is falseFormat

（SomeClass）aObjectVariable

(1) The target type of object variable conversion must be Subclasses of the current object type

(2) Object type checking must also be performed at run time

Object class

Every class in java is a direct or indirect subclass of the Object class.

equals class: compare the values ​​of two objects

Override the equals() method Purpose: define the value of the object

Java regulations: two The hashCode() return values ​​of objects with equal values ​​must be equal, so override the equals() method and also the hashCode() method, using "==" to compare the addresses of the two objects.​ ​ ​

toString() method

Returns the string representation of the object.

getClass() method

Returns the class information of the object. This method returns an object of Class type.

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